The use of N-(carboxymethylaminocarbonyl)-4,4′-bis(dimethylamino) diphenylamine sodium salt as a highly sensitive color former that develops color by oxidation has been known generally. Such a highly sensitive color former can be used, for example, when determining the amount of an oxidizing substance by causing a reaction between the color former and the oxidizing substance using an oxidoreductase and then measuring an amount of the color developed by measuring an absorbance. When N-(carboxymethyl aminocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt is to be used in such a redox reaction, a solution prepared by dissolving it in water usually is used as a liquid reagent.
However, an oxidation color former such as N-(carboxymethylamino carbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt is unstable in an aqueous solution and thus may develop color spontaneously within a day after the preparation of the aqueous solution. Therefore, when N-(carboxy methylaminocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt stored as a solution is used, there arises a problem in that a background absorbance increases in the measurement of an absorbance, thereby degrading the accuracy of the measurement.
In order to prevent the influence of such spontaneous color development, it is necessary to prepare a liquid reagent for each measurement. However, this makes the operation complicated and results in high cost.